A vehicle position determination system and method is described in U.S. Pat. No. 6,219,613, which is owned in common with the present application. The vehicle position determination system described therein determines the position of a vehicle in an open-road ETC system by counting the number of interrogation-response communications per antenna. Subject to some weighting, the antenna with the highest count is associated with the position of the transponder-equipped vehicle.
The described system makes its determination following expiry of a sampling time period, which is preset based upon the interrogation cycle time, the roadway speed limit, and various other factors. The sampling time period is set so as to allow the vehicle, under normal conditions, to traverse a significant portion of the coverage zone before the determination is made. If the vehicle is travelling at a slower-than-expected speed and only traverses a small distance into the zone, then the lane assignment may be incorrect and consequent problems with electronic toll transactions or enforcement may result.
In another embodiment, the sampling time period expires when the transponder-equipped vehicle no longer responds to any interrogations—i.e. when it leaves the coverage zone. In many circumstances it is advantageous to make a determination as to lane position for a vehicle before it leaves the coverage zone.
In addition to vehicle position determination, other sub-systems of the ETC system may operate on the basis of a preset time period, which is established based upon assumptions regarding vehicle travel time. For example, an in-ground loop detector system for determining the number of axles on a passing vehicle bases its decision on the number of axles detected within a certain time period. The time period takes into account the expected speed of the vehicles. If the vehicles are travelling much slower than expected, then the loop detector system may make an incorrect determination. Similarly, enforcement systems within the ETC system, like overhead cameras, may by triggered to operate when a vehicle passing through the communication zone may be expected to pass through the camera viewing field. The timing for operation of the camera may be partly based upon expected vehicle travel time from a detection point. Vehicles travelling at a slower than expected speed may not come within the field of view when expected.
Therefore, it would be advantageous to provide for an ETC system that addresses, at least in part, some of these issues.